Merge tag 'trace-printf-v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/trace...

[drm/drm-misc.git] / drivers / cpuidle / governors / teo.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Timer events oriented CPU idle governor
 *
 * Copyright (C) 2018 - 2021 Intel Corporation
 * Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
 */

/**
 * DOC: teo-description
 *
 * The idea of this governor is based on the observation that on many systems
 * timer events are two or more orders of magnitude more frequent than any
 * other interrupts, so they are likely to be the most significant cause of CPU
 * wakeups from idle states.  Moreover, information about what happened in the
 * (relatively recent) past can be used to estimate whether or not the deepest
 * idle state with target residency within the (known) time till the closest
 * timer event, referred to as the sleep length, is likely to be suitable for
 * the upcoming CPU idle period and, if not, then which of the shallower idle
 * states to choose instead of it.
 *
 * Of course, non-timer wakeup sources are more important in some use cases
 * which can be covered by taking a few most recent idle time intervals of the
 * CPU into account.  However, even in that context it is not necessary to
 * consider idle duration values greater than the sleep length, because the
 * closest timer will ultimately wake up the CPU anyway unless it is woken up
 * earlier.
 *
 * Thus this governor estimates whether or not the prospective idle duration of
 * a CPU is likely to be significantly shorter than the sleep length and selects
 * an idle state for it accordingly.
 *
 * The computations carried out by this governor are based on using bins whose
 * boundaries are aligned with the target residency parameter values of the CPU
 * idle states provided by the %CPUIdle driver in the ascending order.  That is,
 * the first bin spans from 0 up to, but not including, the target residency of
 * the second idle state (idle state 1), the second bin spans from the target
 * residency of idle state 1 up to, but not including, the target residency of
 * idle state 2, the third bin spans from the target residency of idle state 2
 * up to, but not including, the target residency of idle state 3 and so on.
 * The last bin spans from the target residency of the deepest idle state
 * supplied by the driver to infinity.
 *
 * Two metrics called "hits" and "intercepts" are associated with each bin.
 * They are updated every time before selecting an idle state for the given CPU
 * in accordance with what happened last time.
 *
 * The "hits" metric reflects the relative frequency of situations in which the
 * sleep length and the idle duration measured after CPU wakeup fall into the
 * same bin (that is, the CPU appears to wake up "on time" relative to the sleep
 * length).  In turn, the "intercepts" metric reflects the relative frequency of
 * situations in which the measured idle duration is so much shorter than the
 * sleep length that the bin it falls into corresponds to an idle state
 * shallower than the one whose bin is fallen into by the sleep length (these
 * situations are referred to as "intercepts" below).
 *
 * In order to select an idle state for a CPU, the governor takes the following
 * steps (modulo the possible latency constraint that must be taken into account
 * too):
 *
 * 1. Find the deepest CPU idle state whose target residency does not exceed
 *    the current sleep length (the candidate idle state) and compute 2 sums as
 *    follows:
 *
 *    - The sum of the "hits" and "intercepts" metrics for the candidate state
 *      and all of the deeper idle states (it represents the cases in which the
 *      CPU was idle long enough to avoid being intercepted if the sleep length
 *      had been equal to the current one).
 *
 *    - The sum of the "intercepts" metrics for all of the idle states shallower
 *      than the candidate one (it represents the cases in which the CPU was not
 *      idle long enough to avoid being intercepted if the sleep length had been
 *      equal to the current one).
 *
 * 2. If the second sum is greater than the first one the CPU is likely to wake
 *    up early, so look for an alternative idle state to select.
 *
 *    - Traverse the idle states shallower than the candidate one in the
 *      descending order.
 *
 *    - For each of them compute the sum of the "intercepts" metrics over all
 *      of the idle states between it and the candidate one (including the
 *      former and excluding the latter).
 *
 *    - If each of these sums that needs to be taken into account (because the
 *      check related to it has indicated that the CPU is likely to wake up
 *      early) is greater than a half of the corresponding sum computed in step
 *      1 (which means that the target residency of the state in question had
 *      not exceeded the idle duration in over a half of the relevant cases),
 *      select the given idle state instead of the candidate one.
 *
 * 3. By default, select the candidate state.
 */

#include <linux/cpuidle.h>
#include <linux/jiffies.h>
#include <linux/kernel.h>
#include <linux/sched/clock.h>
#include <linux/tick.h>

#include "gov.h"

/*
 * The PULSE value is added to metrics when they grow and the DECAY_SHIFT value
 * is used for decreasing metrics on a regular basis.
 */
#define PULSE           1024
#define DECAY_SHIFT     3

/**
 * struct teo_bin - Metrics used by the TEO cpuidle governor.
 * @intercepts: The "intercepts" metric.
 * @hits: The "hits" metric.
 */
struct teo_bin {
        unsigned int intercepts;
        unsigned int hits;
};

/**
 * struct teo_cpu - CPU data used by the TEO cpuidle governor.
 * @time_span_ns: Time between idle state selection and post-wakeup update.
 * @sleep_length_ns: Time till the closest timer event (at the selection time).
 * @state_bins: Idle state data bins for this CPU.
 * @total: Grand total of the "intercepts" and "hits" metrics for all bins.
 * @tick_hits: Number of "hits" after TICK_NSEC.
 */
struct teo_cpu {
        s64 time_span_ns;
        s64 sleep_length_ns;
        struct teo_bin state_bins[CPUIDLE_STATE_MAX];
        unsigned int total;
        unsigned int tick_hits;
};

static DEFINE_PER_CPU(struct teo_cpu, teo_cpus);

/**
 * teo_update - Update CPU metrics after wakeup.
 * @drv: cpuidle driver containing state data.
 * @dev: Target CPU.
 */
static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
{
        struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
        int i, idx_timer = 0, idx_duration = 0;
        s64 target_residency_ns;
        u64 measured_ns;

        if (cpu_data->time_span_ns >= cpu_data->sleep_length_ns) {
                /*
                 * One of the safety nets has triggered or the wakeup was close
                 * enough to the closest timer event expected at the idle state
                 * selection time to be discarded.
                 */
                measured_ns = U64_MAX;
        } else {
                u64 lat_ns = drv->states[dev->last_state_idx].exit_latency_ns;

                /*
                 * The computations below are to determine whether or not the
                 * (saved) time till the next timer event and the measured idle
                 * duration fall into the same "bin", so use last_residency_ns
                 * for that instead of time_span_ns which includes the cpuidle
                 * overhead.
                 */
                measured_ns = dev->last_residency_ns;
                /*
                 * The delay between the wakeup and the first instruction
                 * executed by the CPU is not likely to be worst-case every
                 * time, so take 1/2 of the exit latency as a very rough
                 * approximation of the average of it.
                 */
                if (measured_ns >= lat_ns)
                        measured_ns -= lat_ns / 2;
                else
                        measured_ns /= 2;
        }

        cpu_data->total = 0;

        /*
         * Decay the "hits" and "intercepts" metrics for all of the bins and
         * find the bins that the sleep length and the measured idle duration
         * fall into.
         */
        for (i = 0; i < drv->state_count; i++) {
                struct teo_bin *bin = &cpu_data->state_bins[i];

                bin->hits -= bin->hits >> DECAY_SHIFT;
                bin->intercepts -= bin->intercepts >> DECAY_SHIFT;

                cpu_data->total += bin->hits + bin->intercepts;

                target_residency_ns = drv->states[i].target_residency_ns;

                if (target_residency_ns <= cpu_data->sleep_length_ns) {
                        idx_timer = i;
                        if (target_residency_ns <= measured_ns)
                                idx_duration = i;
                }
        }

        /*
         * If the deepest state's target residency is below the tick length,
         * make a record of it to help teo_select() decide whether or not
         * to stop the tick.  This effectively adds an extra hits-only bin
         * beyond the last state-related one.
         */
        if (target_residency_ns < TICK_NSEC) {
                cpu_data->tick_hits -= cpu_data->tick_hits >> DECAY_SHIFT;

                cpu_data->total += cpu_data->tick_hits;

                if (TICK_NSEC <= cpu_data->sleep_length_ns) {
                        idx_timer = drv->state_count;
                        if (TICK_NSEC <= measured_ns) {
                                cpu_data->tick_hits += PULSE;
                                goto end;
                        }
                }
        }

        /*
         * If the measured idle duration falls into the same bin as the sleep
         * length, this is a "hit", so update the "hits" metric for that bin.
         * Otherwise, update the "intercepts" metric for the bin fallen into by
         * the measured idle duration.
         */
        if (idx_timer == idx_duration)
                cpu_data->state_bins[idx_timer].hits += PULSE;
        else
                cpu_data->state_bins[idx_duration].intercepts += PULSE;

end:
        cpu_data->total += PULSE;
}

static bool teo_state_ok(int i, struct cpuidle_driver *drv)
{
        return !tick_nohz_tick_stopped() ||
                drv->states[i].target_residency_ns >= TICK_NSEC;
}

/**
 * teo_find_shallower_state - Find shallower idle state matching given duration.
 * @drv: cpuidle driver containing state data.
 * @dev: Target CPU.
 * @state_idx: Index of the capping idle state.
 * @duration_ns: Idle duration value to match.
 * @no_poll: Don't consider polling states.
 */
static int teo_find_shallower_state(struct cpuidle_driver *drv,
                                    struct cpuidle_device *dev, int state_idx,
                                    s64 duration_ns, bool no_poll)
{
        int i;

        for (i = state_idx - 1; i >= 0; i--) {
                if (dev->states_usage[i].disable ||
                                (no_poll && drv->states[i].flags & CPUIDLE_FLAG_POLLING))
                        continue;

                state_idx = i;
                if (drv->states[i].target_residency_ns <= duration_ns)
                        break;
        }
        return state_idx;
}

/**
 * teo_select - Selects the next idle state to enter.
 * @drv: cpuidle driver containing state data.
 * @dev: Target CPU.
 * @stop_tick: Indication on whether or not to stop the scheduler tick.
 */
static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
                      bool *stop_tick)
{
        struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
        s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
        ktime_t delta_tick = TICK_NSEC / 2;
        unsigned int tick_intercept_sum = 0;
        unsigned int idx_intercept_sum = 0;
        unsigned int intercept_sum = 0;
        unsigned int idx_hit_sum = 0;
        unsigned int hit_sum = 0;
        int constraint_idx = 0;
        int idx0 = 0, idx = -1;
        int prev_intercept_idx;
        s64 duration_ns;
        int i;

        if (dev->last_state_idx >= 0) {
                teo_update(drv, dev);
                dev->last_state_idx = -1;
        }

        cpu_data->time_span_ns = local_clock();
        /*
         * Set the expected sleep length to infinity in case of an early
         * return.
         */
        cpu_data->sleep_length_ns = KTIME_MAX;

        /* Check if there is any choice in the first place. */
        if (drv->state_count < 2) {
                idx = 0;
                goto out_tick;
        }

        if (!dev->states_usage[0].disable)
                idx = 0;

        /* Compute the sums of metrics for early wakeup pattern detection. */
        for (i = 1; i < drv->state_count; i++) {
                struct teo_bin *prev_bin = &cpu_data->state_bins[i-1];
                struct cpuidle_state *s = &drv->states[i];

                /*
                 * Update the sums of idle state mertics for all of the states
                 * shallower than the current one.
                 */
                intercept_sum += prev_bin->intercepts;
                hit_sum += prev_bin->hits;

                if (dev->states_usage[i].disable)
                        continue;

                if (idx < 0)
                        idx0 = i; /* first enabled state */

                idx = i;

                if (s->exit_latency_ns <= latency_req)
                        constraint_idx = i;

                /* Save the sums for the current state. */
                idx_intercept_sum = intercept_sum;
                idx_hit_sum = hit_sum;
        }

        /* Avoid unnecessary overhead. */
        if (idx < 0) {
                idx = 0; /* No states enabled, must use 0. */
                goto out_tick;
        }

        if (idx == idx0) {
                /*
                 * Only one idle state is enabled, so use it, but do not
                 * allow the tick to be stopped it is shallow enough.
                 */
                duration_ns = drv->states[idx].target_residency_ns;
                goto end;
        }

        tick_intercept_sum = intercept_sum +
                        cpu_data->state_bins[drv->state_count-1].intercepts;

        /*
         * If the sum of the intercepts metric for all of the idle states
         * shallower than the current candidate one (idx) is greater than the
         * sum of the intercepts and hits metrics for the candidate state and
         * all of the deeper states a shallower idle state is likely to be a
         * better choice.
         */
        prev_intercept_idx = idx;
        if (2 * idx_intercept_sum > cpu_data->total - idx_hit_sum) {
                int first_suitable_idx = idx;

                /*
                 * Look for the deepest idle state whose target residency had
                 * not exceeded the idle duration in over a half of the relevant
                 * cases in the past.
                 *
                 * Take the possible duration limitation present if the tick
                 * has been stopped already into account.
                 */
                intercept_sum = 0;

                for (i = idx - 1; i >= 0; i--) {
                        struct teo_bin *bin = &cpu_data->state_bins[i];

                        intercept_sum += bin->intercepts;

                        if (2 * intercept_sum > idx_intercept_sum) {
                                /*
                                 * Use the current state unless it is too
                                 * shallow or disabled, in which case take the
                                 * first enabled state that is deep enough.
                                 */
                                if (teo_state_ok(i, drv) &&
                                    !dev->states_usage[i].disable)
                                        idx = i;
                                else
                                        idx = first_suitable_idx;

                                break;
                        }

                        if (dev->states_usage[i].disable)
                                continue;

                        if (!teo_state_ok(i, drv)) {
                                /*
                                 * The current state is too shallow, but if an
                                 * alternative candidate state has been found,
                                 * it may still turn out to be a better choice.
                                 */
                                if (first_suitable_idx != idx)
                                        continue;

                                break;
                        }

                        first_suitable_idx = i;
                }
        }
        if (!idx && prev_intercept_idx) {
                /*
                 * We have to query the sleep length here otherwise we don't
                 * know after wakeup if our guess was correct.
                 */
                duration_ns = tick_nohz_get_sleep_length(&delta_tick);
                cpu_data->sleep_length_ns = duration_ns;
                goto out_tick;
        }

        /*
         * If there is a latency constraint, it may be necessary to select an
         * idle state shallower than the current candidate one.
         */
        if (idx > constraint_idx)
                idx = constraint_idx;

        /*
         * Skip the timers check if state 0 is the current candidate one,
         * because an immediate non-timer wakeup is expected in that case.
         */
        if (!idx)
                goto out_tick;

        /*
         * If state 0 is a polling one, check if the target residency of
         * the current candidate state is low enough and skip the timers
         * check in that case too.
         */
        if ((drv->states[0].flags & CPUIDLE_FLAG_POLLING) &&
            drv->states[idx].target_residency_ns < RESIDENCY_THRESHOLD_NS)
                goto out_tick;

        duration_ns = tick_nohz_get_sleep_length(&delta_tick);
        cpu_data->sleep_length_ns = duration_ns;

        /*
         * If the closest expected timer is before the target residency of the
         * candidate state, a shallower one needs to be found.
         */
        if (drv->states[idx].target_residency_ns > duration_ns) {
                i = teo_find_shallower_state(drv, dev, idx, duration_ns, false);
                if (teo_state_ok(i, drv))
                        idx = i;
        }

        /*
         * If the selected state's target residency is below the tick length
         * and intercepts occurring before the tick length are the majority of
         * total wakeup events, do not stop the tick.
         */
        if (drv->states[idx].target_residency_ns < TICK_NSEC &&
            tick_intercept_sum > cpu_data->total / 2 + cpu_data->total / 8)
                duration_ns = TICK_NSEC / 2;

end:
        /*
         * Allow the tick to be stopped unless the selected state is a polling
         * one or the expected idle duration is shorter than the tick period
         * length.
         */
        if ((!(drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
            duration_ns >= TICK_NSEC) || tick_nohz_tick_stopped())
                return idx;

        /*
         * The tick is not going to be stopped, so if the target residency of
         * the state to be returned is not within the time till the closest
         * timer including the tick, try to correct that.
         */
        if (idx > idx0 &&
            drv->states[idx].target_residency_ns > delta_tick)
                idx = teo_find_shallower_state(drv, dev, idx, delta_tick, false);

out_tick:
        *stop_tick = false;
        return idx;
}

/**
 * teo_reflect - Note that governor data for the CPU need to be updated.
 * @dev: Target CPU.
 * @state: Entered state.
 */
static void teo_reflect(struct cpuidle_device *dev, int state)
{
        struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);

        dev->last_state_idx = state;
        /*
         * If the wakeup was not "natural", but triggered by one of the safety
         * nets, assume that the CPU might have been idle for the entire sleep
         * length time.
         */
        if (dev->poll_time_limit ||
            (tick_nohz_idle_got_tick() && cpu_data->sleep_length_ns > TICK_NSEC)) {
                dev->poll_time_limit = false;
                cpu_data->time_span_ns = cpu_data->sleep_length_ns;
        } else {
                cpu_data->time_span_ns = local_clock() - cpu_data->time_span_ns;
        }
}

/**
 * teo_enable_device - Initialize the governor's data for the target CPU.
 * @drv: cpuidle driver (not used).
 * @dev: Target CPU.
 */
static int teo_enable_device(struct cpuidle_driver *drv,
                             struct cpuidle_device *dev)
{
        struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);

        memset(cpu_data, 0, sizeof(*cpu_data));

        return 0;
}

static struct cpuidle_governor teo_governor = {
        .name =         "teo",
        .rating =       19,
        .enable =       teo_enable_device,
        .select =       teo_select,
        .reflect =      teo_reflect,
};

static int __init teo_governor_init(void)
{
        return cpuidle_register_governor(&teo_governor);
}

postcore_initcall(teo_governor_init);